The present invention relates to a color conversion filter substrate with good environmental resistance and high productivity for displaying multiple colors with high definition, and to an organic multicolor emitting display device provided with such a filter substrate. More specifically, the present invention relates to a color conversion filter substrate and an organic multicolor emitting display device provided with such a filter substrate for a display of electronic and electric equipment such as an image sensor, a personal computer, a word processor, a television, a fax machine, an audio equipment, a video equipment, a car navigation system, an electric desk top calculator, a telephone, a portable terminal, or an industrial instrument.
In recent years, the information technology has been diversified. Among elements used in the information technology, display devices including solid imaging devices have been required to have a better aesthetic appearance, a lighter weight, a thinner thickness and higher performance. Furthermore, a great deal of effort has been made to reduce power consumption and increase a response speed. In particular, many attempts have been made to develop high-definition full-color display devices.
In the second half of the 1980s, Tang et al. reported that an organic electro-luminescence (hereinafter referred as an “organic EL”) formed of stacked thin films of organic molecules showed a high luminance of 1000 cd/m2 at an applied voltage of 10 V (Appl. Phys. Lett., 51, 913 (1987)). This stacked organic EL device has excellent characteristics such as a wide view angle and a quick response time compared to liquid crystal display devices. After the report by Tang et al., a great effort has been made to develop organic EL devices for a practical use. Attempts have also been made to develop similar devices composed of an organic polymer material.
Since the organic EL device provides a high current density at a low voltage, it is expected to provide higher emission luminance and efficiency as opposed to inorganic EL devices and LEDs.
The organic EL display device is expected to have characteristics such as (1) high luminance and high contrast, (2) low driving voltage and high emission efficiency, (3) high resolution, (4) wide angle visibility, (5) high response speed, (6) possibility of increasing definition and providing color displays, (7) reduced weight and reduced thickness, and the like. Thus, the organic EL device is expected to have a better aesthetic appearance, a lighter weight, a thinner thickness and higher performance.
Tohoku Pioneer Corporation has already developed products including vehicle-mounted green monochrome organic EL displays since November 1997. In order to meet the society needs, it is desirable to develop improved organic EL displays that are stable for an extended period of time, respond quickly, and display multiple colors or full colors with high definition.
A method of displaying multiple or full colors with the organic EL display includes methods disclosed in Japanese Patent Publications No. 57-167487, No. 58-147989, and No. 03-214593, in which light emitting elements of the three primary colors (red, green, and blue) are arranged in a matrix form. In a case that the organic light emitting device is used to provide a multicolor display, it is necessary to arrange three types of light-emitting materials (R, G, and B) in a matrix form with high precision, thereby making it technically difficult to produce and increasing a cost. Further, the three types of light-emitting materials have different life times, thereby shifting a color of the display with time.
In Japanese Patent Publications No. 01-315988, No. 02-273496, and No. 03-194895, a method in which a color filter and a backlight emitting white light are used to display the three primary colors through the filter has been disclosed. However, it is difficult to obtain an organic light emitting device emitting the white light with a long life, which is necessary for obtaining bright three colors R, G, and B.
Japanese Patent Publication No. 03-152897 disclosed another method in which phosphors arranged on a plane absorb light from light emitting devices, so that the phosphors emit fluorescence in multiple colors. Such a method using a certain luminous device to allow the phosphors to emit fluorescence in multiple colors has been applied to CRTs, plasma displays, and the like.
Further, in recent years, a color conversion method has been proposed in which a filter is composed of a fluorescent material for absorbing light with a wavelength in a light-emission region of an organic light emitting device, so that the fluorescent material emits fluorescence with a wavelength in a visible light region (Japanese Patent Publications No. 03-152897 and No. 05-258860). In this approach, an organic light emitting device that emits a color other than white can be used. Therefore, it is possible to use an organic light emitting device with higher brightness as a light source. In a color conversion method using an organic light emitting device emitting blue light (Japanese Patent Publications No. 03-152897, No. 08-286033, and No. 09-208944), a frequency of blue light is converted to that of green or red light. A color conversion filter containing a fluorescent material with such color conversion effect may be formed in a high resolution pattern. Accordingly, it is possible to provide a full-color light emitting display even with weak energy light such as near-ultraviolet light or visible light.
In order to form a pattern of a color conversion filter, a method in which a pattern is formed with a photolithography process after a film of a resist (photosensitive polymer) material containing fluorescent material is prepared by spin-coating is disclosed in Japanese Patent Publications No. 05-198921 and No. 05-258860. Also, Japanese Patent Publication No. 09-208944 discloses a process in which a fluorescent material or fluorescent pigment is dispersed in a basic binder followed by etching the binder with an acid solution.
In general, it is important for a practical color display to possess high-resolution color and long-term stability (as described in KinohZairyo Vol. 18, No. 2, 96). However, the organic EL devices tend to markedly lose light-emission characteristics such as current-luminance characteristics after a specific period of time.
A major cause of the degraded light-emission characteristics is a growth of dark spots in the light-emitting layer. The dark spots are formed of light-emission defects. When the fluorescent material in the light-emitting layer is oxidized while using or storing the organic EL device, the dark spots grow and spread over the entire light-emitting surface.
It is believed that the dark spots are created by oxidation or aggregation of a material constituting a layered device caused by oxygen or moisture in the device. The dark spots grow not only when electricity is conducted but also during storage. In particular, it is believed that (i) the growth is accelerated by oxygen or moisture present around the device, (ii) the growth is affected by oxygen or moisture attached to the organic stacked films, and (iii) the growth is affected by moisture attached to parts or entered in the device when the device is manufactured.
Thus, methods have been proposed to suppress the growth of the dark spots. For example, a method in which a device is installed and sealed in an airtight container along with diphosphorous pentaoxide as a desiccant for drying the layered material of the device is disclosed in Japanese Patent Publication No. 03-261091. Also, Japanese Patent Publication No. 07-169567 discloses a structure in which diphosphorous pentaoxide is blended into a protective layer and a sealing layer for drying. In these methods, however, diphosphorous pentaoxide for drying is changed into phosphoric acid due to the moisture, thereby affecting the organic layered material. Other proposed methods include an airtight container filled with an inactive liquid containing a desiccant (Japanese Patent Publications No. 05-4128 and No. 09-35868), and use of a pressure-sensitive adhesive (U.S. Pat. No. 5,304,419).
As described above, in order to provide an organic multicolor light emitting device with stable light-emission characteristics for an extended period of time, the growth of the dark spots needs to be sufficiently suppressed.
FIG. 1 shows an example of the layered structure of the color conversion filter substrate. In the figure, reference numerals 1, 2, 3, 4, 5, and 6 refer to a transparent support substrate, a red conversion filter, a green conversion filter, a blue conversion filter, a first gas-barrier layer, and a second gas-barrier layer, respectively. As shown in FIG. 2, an organic multicolor light emitting device includes such a color conversion substrate. The organic multicolor light emitting device is constructed by sequentially stacking a transparent electrode 7, a hole-injection layer 8, a hole-transporting layer 9, an organic light-emitting layer 10, an electron-injection layer 11, and an electrode 12 on the color conversion filter substrate.
As shown in FIG. 2, in the color conversion organic multicolor light emitting device (organic EL display), the color conversion filters 2, 3, and 4 are disposed under the transparent electrode 7. As described above, the color conversion filter is formed of the resin containing the colorant for color conversion. Because of thermal stability of the colorant, it is not possible to dry the color conversion filter at a temperature above 200° C. Accordingly, it is likely that the color conversion filters 2, 3, and 4 contain moisture from a coating liquid or entered during a pattern-forming process. The moisture in the color conversion filters 2, 3, and 4 passes through the barrier layers 5 and 6 to the light-emitting layer 10 while the device is stored or is continuously operated, thereby facilitating the growth of the dark spots in the light-emitting layer 10.
The sealing method described above is to prevent moisture and oxygen in the air from entering the light emitting device. Thus, with this method, it is difficult to eliminate the moisture from the color conversion filters, thereby making it difficult to successfully develop a color organic EL display with the stable light-emission characteristics for an extended period of time.
The present invention is provided in view of the above problems. An object of the present invention is to provide a color conversion filter substrate with stable light-emission characteristics for an extended period of time, in which moisture is suppressed to prevent degradation of the organic multicolor light emitting device.
Further objects and advantages of the invention will be apparent from the following description of the invention.